Acoustic sandwich panels, comprising two skins bounding a core structure (such as honeycomb core), are common building blocks used in aerospace and other fields. They typically comprise two skin surfaces which sandwich between them at least one layer of a core material. Typically the two skins and the core may be bonded together or cured/formed together, but mechanical fastening is also used in some applications. The core ties the skins together structurally, and can form a very rigid, efficient, and lightweight structure useful in aerospace applications. In many applications, the acoustic sandwich panel serves a structural role in addition to an acoustic role, but in some applications the structural function may be secondary to the acoustics. In aerospace, nacelles which house turbofan gas turbine engines on a commercial transport airplane may use acoustic sandwich panels to form the inner barrel of the inlet, the inner fixed structure of the thrust reverser, the translating sleeve of a translating sleeve type thrust reverser, and portions of an exhaust system.
In some applications, there is a tension between the acoustic attenuation requirements for an acoustic sandwich panel, and the structural and packaging requirements. For example, to attenuate an increasing pressure level of low frequency noise from modern high-bypass-ratio turbofan engines (because fans are increasingly larger and slower turning), an acoustic sandwich panel may require larger cavities that are tuned to these lower frequencies. But constructing a typical acoustic sandwich panel with increasingly larger cavities may not allow the panel to fulfill its requirements related to structural capability, and/or may not be possible within the packaging, size, and weight constraints.
The invention herein allows for a new balance between a panel's acoustic capabilities, especially at the lower end of the frequency range, and its structural capabilities and packaging requirements.